The present invention relates to spectroscopic analysis of materials, and particularly, to non-contact, remote spectroscopic analysis of a quantity of flowable material based on transient thermal infrared emission from the material. The flowable material includes liquids, gases, melts representing normally solid materials which have been heated above the melting temperature thereof, powders and pellets and which are enclosed in a transport system such as a container, conduit or the like.
There are numerous types of analytical methods which currently are known for deriving information about materials. Spectroscopy is a well known and general method for analyzing materials. There are a number of types of spectroscopic methods which, in turn, are applicable to certain types of analyses and measurements, and which have advantages and disadvantages.
Presently, there is a need for improvements in the ability to analyze materials, especially in those cases where such analyses need to be quick, efficient, and accurate. Additionally, there is a real need for such analyses for "in-process" situations; that is, directly on-line with respect to the manufacturing or the processing of materials.
For many materials, there are a variety of generally conventional spectroscopic methods for analyzing the content and other characteristics of the materials. Some of those methods are infrared transmission, diffuse reflectance, photoacoustic, and emission spectroscopies. While generally these methods give satisfactory results, they are deficient because they require selective, and often destructive, sampling of the materials. Some materials (coal, for example) require grinding or pulverizing. The material must often be removed to a remote laboratory location where the testing and equipment requires time and resources to provide the results.
Many of the aforementioned presently used methods also lack much flexibility in their use. While some of the methods do not require destructive sampling such as grinding or pulverizing, they may not be operable for materials of greater than minimal thickness, or for materials of varying thickness. Conventional transmission or emission spectroscopies have problems because the optical density of many materials is too high to permit accurate and reliable measurement. When a thick sample is heated, the deep layers of the sample emit strongly at the preferred wavelengths and only weakly at other wavelengths. This deep-layer strong emission at preferred wavelengths, however, is greatly attenuated before leaving the sample since surface layers of the thick sample preferentially absorb those particular wavelengths and such process is termed "self-absorption". Self-absorption in optically-thick samples causes severe truncation of strong spectroscopic bands and leads to emission spectra which closely resemble black-body emission spectra representative of an optically thick material being heated to a uniform temperature and which contain little spectral structure characteristic of the material being analyzed.
Attempts have been made to solve this self-absorption problem by thinning sample materials. High-quality spectra of free-standing films and thin layers on low-emission substrates are routinely measured. However, this requires selective sampling and processing of the materials being analyzed.
For other types of spectroscopic methods such as photoacoustic and reflection spectroscopies which are less subject to optical density problems, deficiencies exist in that they are not easily performed on moving streams of materials. Thus, there is a real need in the art for an apparatus and method which has the flexibility to be used both for moving and stationary materials; and for materials which may have significant optical densities.
In the aforementioned copending applications, transient infrared spectroscopy has been applied to materials wherein a temperature gradient is created at the surface of the analyzed material so that a thin surface layer of the material is hotter or colder than the bulk of the sample material. If the surface is heated, it emits infrared light independent of the bulk of the sample material. Since the surface layer is thin, the spectrum thereof suffers less from the phenomenon of self-absorption that obscures the emission spectra of optically thick materials and can be appropriately detected to provide an indication of characteristics relating to molecular composition of the sample material. In a similar manner, if the surface is cooled, the surface layer acts as a thin transmission sample and the transmission spectrum which is detected is impressed onto the infrared light passing through the thin surface layer that is spontaneously emitted by the uncooled bulk of the material sample to provide an indication of characteristics relating to molecular composition of the sample material. However, the prior copending applications are generally related to materials which are not enclosed in a transport system such as a container, conduit, or the like.